The present invention relates to a semiconductor integrated circuit device, and more particularly to a protective circuit element.
FIG. 3 is a section view of a semiconductor chip, showing one example of a conventional protective off-buffer element. Referring to FIG. 3, a high voltage system (power supply voltage of 10 V or higher) is composed of a field oxidized film 2 which is provided in the surface of a P type silicon substrate 1 for plotting an element forming region, a p type guard ring layer 3 which is provided below the field oxidized film 2, a gate oxidized film 4 which is formed in the surface of the element forming region, a gate electrode 11 which is selectively formed on the gate oxidized film 4, source and drain regions each having a dual diffusion layer structure, which is composed of a deep n.sup.- type diffusion layer 6 formed by implanting impure ions in the element forming region with this gate electrode 11 and the field oxidized film 2 as masks and a shallow n.sup.+ type diffusion layer 7 provided inside the n.sup.- type diffusion layer 6, a layer insulating film 8 which is provided in a surface including the gate electrode 11, and drain and source electrodes 12 and 13 which are formed by being connected to the n.sup.+ type diffusion layer 6 of a contact hole provided in the layer insulating film 8.
This configuration is the same as that of a logic circuit high voltage system MOS transistor, and as shown in the equivalent circuit of FIG. 4, this high voltage system is used as an off-buffer by keeping the gate electrode at the same potential as that of the substrate.
If this off-buffer is used as a protective element, the sequence of its operational steps is as follows.
(1) A high voltage pulse is impressed to the drain electrode.
(2) A breakdown occurs in the junction diode D of the drain diffusion layer.
(3) A hole created as an electron and hole pair raises a potential under the gate electrode.
(4) A parasitic npn type bipolar transistor B is switched ON, and thereby resistance after the breakdown can be reduced.
In this case, a current flows through drain and source resistors R.sub.D and R.sub.S, and thus heat generating phenomena occur in the drain and source resistors R.sub.D and R.sub.S
There were problems inherent in this conventional semiconductor integrated circuit device. More particularly, since the structure of the protective circuit element was the same as that of the internal logic circuit, its breakdown voltage was also the same. Furthermore, when the densities of the n.sup.- type diffusion layers of the drain and source regions were low and drain resistance and source resistance were high, switching-ON of the parasitic npn type bipolar transistor caused an overcurrent to flow, and heating of the drain and source regions brought about junction destruction.
Accordingly, when the off-buffer was to be used as a protective element, it was necessary to disperse a current by increasing its occupancy area.